1. Field of the Invention
The invention lies in the field of electro-optical communications. The invention relates to an interference-robust coded-modulation scheme for optical communications and a method for modulating illumination for optical communications.
2. Description of the Related Art
In the recent past, the Federal Communications Commission (xe2x80x9cFCCxe2x80x9d) has promulgated standards related to the Emergency-911 (xe2x80x9cE-911xe2x80x9d) initiative. The initiative has set the initial standards for a communications system that, in the future, will precisely locate a particular person or object. Particularly, the initiative relates to precisely determining the location of a particular cellular telephone. As a result, location technologies have been developing rapidly. The rush to complete E-911, in particular, has compelled many cellular system designers to develop a scheme that will satisfy the location requirements set by the FCC.
Simultaneously, smaller-scale location opportunities, like personal security and asset-management solutions, are being investigated. The smaller-scale location technologies present many more unsolved problems than the large-scale E-911 technology. One of the most challenging feats resides in the creation of technology that is able to locate a device within structures. As set forth herein, the terms xe2x80x9cstructurexe2x80x9d or xe2x80x9cstructuresxe2x80x9d mean any enclosed space having intermediate areas and walls. The terms do not necessarily include only man-made structures, such as buildings, for example. The term xe2x80x9careasxe2x80x9d as used herein is defined as any subset of the enclosed space within a structure. An xe2x80x9careaxe2x80x9d can mean, for example, a small room with four walls and a door, or it can refer to a large room with many walls and doors and with intermediate cubicle-type half-walls.
Equipment that uses radio frequency (xe2x80x9cRFxe2x80x9d) signals to capture Receive Signal Strength (xe2x80x9cRSSxe2x80x9d), Time Difference of Arrival (xe2x80x9cTDoAxe2x80x9d), or Angle of Arrival (xe2x80x9cAoAxe2x80x9d) clues have limited location accuracy and often require an expensive supporting infrastructure. RF location schemes like RADAR, GPS, and LORAN have been used for years to locate people and/or objects outdoors. The realities of the indoor environment prevent RF schemes from being employed indoors. Specifically, when RF propagates in a building, the transmitted signal undergoes fading, dispersion, and interference with delayed versions of itselfxe2x80x94otherwise known in the art as multipath interference. Such signal impediments make it extremely difficult to configure an indoor RF location system employing RSS, TDOA, or AoA for estimating a location of an object.
In most cases, existing optical in-building infrastructure includes already-installed lights, such as incandescent bulbs, fluorescent lamps, and halogen bulbs, or even LED""s or laser diodes. These definitions for the term xe2x80x9clightxe2x80x9d or xe2x80x9clightsxe2x80x9d are not exclusive. xe2x80x9cLightxe2x80x9d can refer to any device used for visible light illumination or invisible light transmission, including, but not limited to, ultraviolet and infrared. Other existing in-building devices include sensors, RF transceivers, and processors that can perform position estimates based on signal strength or some other ranging technology.
Various prior art devices and methods have used lighting for sending information in addition to providing illumination. The first four paragraphs of the background section of International PCT publication WO 99/53732 to Leeb et al. detail the progression of such devices and methods over the past few decades. These paragraphs are hereby incorporated herein by reference. None of the cited patents, however, provide the features of the invention.
For example, WO 99/53732 discloses an apparatus for modulating electromagnetic radiation to transmit information from a visible-light generating lamp such that human-perceptible flicker is eliminated regardless of the information content.
Additionally, International PCT publication WO 00/30415 to Lupton et al. discloses an electronic communications network that uses indoor fixtures, emitting modulated visible light, as transmitters that do not generate human-perceptible flicker.
Finally, International PCT publication WO 99/53633 to Hovorka et al. discloses a communications network similar to WO 00/30415 having an improved bandwidth using a particular coding scheme.